In order to control a moving object based on the position of the moving object that runs along a one-dimensional curve (hereinafter referred to as a track), for example, a train that runs on a railway track, a method of calculating, based on a speed of the moving object, a moved distance of the moving object along a track and calculating the position of the moving object has been used. For example, as a method for measuring a running position of the train, accumulating values as moved distances, each of which is obtained by multiplying the number of wheel rotations obtained from a tachometer generator by a wheel circumference. Then, with reference to position information of a track antenna provided to a train protection apparatus that is placed on the railway track, the position of the moving object on the track is calculated based on the accumulated moved distance.
With respect to this, measuring a position by use of Global Positioning System (GPS) has been a known technique. For example, Patent Literature 1 discloses a technique for simplifying the measurement of the position by calculating the position based on position information obtained from the GPS and speed information obtained from the tachometer generator. Patent Literature 1 determines, as a current train position, the position obtained by adding, to the position of an own train found based on a GPS signal, a correction value calculated based on a status of the own train, when a reception status value is equal to or greater than a threshold value which is used for determining accuracy of the position of the own train found based on the GPS signal, or alternatively, determines, as the current position of the train, the position obtained by adding the correction value to the position obtained by adding the moved distance to a reference position, when the reception status value is less than the threshold value.
Also, Patent Literature 2 discloses a method of excluding an erroneous positioning result under assumption that a radio wave that causes an error is transmitted from at best no more than one satellite.